A former Navy reservist suspected of killing 12 people in a shooting rampage at a US naval building claimed he was involved in rescuing victims of the September 11 terrorist attacks in New York and had been traumatised by the experience.

The dead gunman has been identified as 34-year-old civilian contractor Aaron Alexis, originally from Fort Worth, Texas. Photographic ID found was found on his body at the US Navy Yard in Washington DC, where 13 people, including Alexis, were killed and at least three people injured.

As police investigated Monday's fatal shooting, just kilometres from the Capitol building and the White House, a picture began to emerge of a troubled man with a history of gun violence.

Died at the scene: the FBI released two photos of suspected shooter Aaron Alexis. Photo: Supplied

The Seattle Police Department confirmed Alexis had been arrested in 2004 after shooting out the tyres of another man's vehicle in what Alexis had described to police was an anger-fuelled “blackout”. He said he had been present during "the tragic events of September 11, 2001" and described "how those events had disturbed him", according to the police report.

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The Seattle Police Department said detectives later spoke with Alexis' father in New York, who told police Alexis had anger-management problems associated with post-traumatic stress disorder.

Alexis' father confirmed that Alexis had been an active participant in rescue attempts on September 11. Police have not revealed further details about Alexis' involvement in helping victims of the terrorist attacks in New York.

Washington shooting

The US Senate was in lockdown and security has been tightened due to a mass shooting at the US Navy Yard in Washington, where at least 12 people were shot and killed by a gunman. It is suspected that one of the gunmen was among the dead and authorities are searching for other possible assailants wearing military-style clothing. Photo: Alex Wong/Getty/AFP

According to the Navy, Alexis enlisted as a full-time reservist in May 2007 and left the service in January 2011.

He served as an aviation electrician, and the highest rank he achieved was mate third class.

From February 2008 to January 2011, he was assigned to Fleet Logistics Support Squadron 46, in Fort Worth.

The Navy said Alexis had been awarded the National Defence Service Medal and the Global War on Terrorism Service Medal.

Friends and acquaintances said Alexis was a quiet, polite man who had divided his time over the past few years between New York, Texas and Washington state.

A man who identified himself as Alexis' "best friend" said he was shocked to learn that Alexis was the suspect.

"He lived with me three years," Nutpisit Suthamtewakul told the Star-Telegramin Forth Worth. "I don't think he'd do this. He has a gun, but I don't think he's that stupid. He didn't seem aggressive to me."

His former landlord in Fort Worth, Somsak Srisan, said Alexis was always well-mannered.

"Oh boy, I can't believe this," Mr Srisan told the newspaper. "He was always very polite to me. When he lived at my house, I never see him get angry about anything. My feeling is if he was angry about anything, he didn't show that to me."

Mr Srisan said he had seen Alexis meditate and he had always remained calm around him.

"I can't believe he would do anything like this," Mr Srisan said.

But police confirmed Alexis had come to police attention at least twice for gun-related incidents.

Police said that the 2004 incident in Seattle had involved two construction workers who had parked their vehicle in the driveway of their work site, next to Alexis' grandmother's home, where Alexis was staying.

The workers reported seeing a man, later identified as Alexis, walk out of the home, pull a gun from his waistband and fire three shots into the two rear tyres of the vehicle, before walking slowly back inside.

When detectives interviewed workers at the site, they said Alexis had “stared” at them every day for a month before the shooting. The construction site manager told police he believed Alexis was angry over the parking situation around the worksite.

Alexis told detectives he believed he had been “mocked” by construction workers the morning of the incident and they had “disrespected him.”

Alexis also claimed he could not remember firing his gun at the vehicle until an hour after the incident.

Six years later, Alexis was arrested in Fort Worth, Texas, for firing a gun through the ceiling of his apartment into his upstairs neighbour's apartment, according to NBC5.

His neighbour called the police after the bullet came through her floor and hit the ceiling.

She told police she was “terrified” of Alexis, who had previously confronted her about making too much noise, according to The Smoking Gun.

The woman, who was “visibly shaken up”, told police that she believed the shooting was “intentional”.

But Alexis claimed he was cleaning the weapon when it discharged, according to The Smoking Gun. He told police his hands were slippery and he had begun "to take the gun apart when his hands slipped and pulled the trigger discharging a round into the ceiling".

Tarrant County District Attorney spokeswoman Melody McDonald told the Dallas Morning News that Alexis had never been charged with a crime in Tarrant County and had been released from Fort Worth city jail the same day he had been arrested.

"We have to find out what kind of case it was, who it went to. But he never had a pending case in this office," she told the newspaper.

“He has never been charged with a crime in Tarrant County, at least not as an adult.”

The Washington Post reported that Alexis appeared to have a government contractor access card that allowed him into the Navy Yard.

Alexis was employed this year as a computer programmer for The Experts, and was working on a large subcontracting job with Hewlett Packard to refresh computer systems at Navy and Marine Corps installations.

Alexis had a security clearance that was updated in July and was approved by military security service personnel, according to the Washington Post.

"Nobody could have done anything to prevent this except Aaron Alexis," Thomas Hoshko, CEO of The Experts, told the newspaper.

"Maybe he snapped. I don't know. It's just the most unfortunate incident I've seen in all my career.''

17 Sep
The US Senate was in lockdown and security has been tightened due to a mass shooting at the US Navy Yard in Washington, where at least 12 people were shot and killed by a gunman. It is suspected that one of the gunmen was among the dead and authorities are searching for other possible assailants wearing military-style clothing.

16 Sep
At least 13 people including a gunman have been killed and a police officer wounded in a mass shooting at a US Navy yard in Washington, DC, just three kilometres from the Capitol building, and police are hunting for one other possible suspect.

17 Sep
The killing at the US Navy Yard began with the work day, as 3000 military and civilian staff were settling at their desks or taking their first meetings in a sprawling facility just three kilometres from the Capitol building.

17 Sep
Aaron Alexis lived for a time in a bungalow in the woods near a Buddhist temple in Fort Worth, Texas, where he occasionally joined Thai immigrants in meditation. Aaron Alexis died on Monday in a gun battle with police in a building at the Washington Navy Yard after he killed at least 12 people.